The present invention provides a system and process for selectively removing organic and inorganic contaminants from plating baths. More particularly, this invention relates to the use of a source of energy in combination with chemical oxidants, alone or in conjunction with a catalyst to oxidize organic contaminants in the plating bath to a level such that the electroplating bath can be recovered and reused after appropriate chemical adjustment.
Plating baths are used to plate thin metal films onto electrical components such as circuit boards and semiconductor wafers. Typical metals used in plating baths include copper, nickel, silver and tin. In semiconductor wafer manufacturing, the formation of consistent high quality thin films of copper is essential to the operation of high-speed microprocessors and memory devices. Copper films in semiconductor devices require electrical resistivity near 1.7 ohm-cm and film thickness near 1 micron. Typical copper plating solutions used for semiconductor processing contain aqueous solutions of sulfuric acid, copper ions, and various organic additives: wetting agents, brighteners, organic acids such as phosphonic and sulfonic acids, and complexing agents. These organic additives are used to achieve high quality, consistent plating. See U.S. Pat. Nos. 5,328,589, 4,110,776, 3,267,010 and 3,770,598. Over time and through use in a plating process, the organic components in the bath tend to degrade or breakdown to form organic contaminants. These organic contaminants are harmful to the plating process because they result in changes in plating efficiency, plating rate, film morphology, film stress, and electrical properties of the plated metal films. Over time and through use in a plating process the bath also accumulates inorganic contaminants that also degrade the plated metal films. The accumulation of organic and inorganic contaminants over time in a plating bath requires that the spent bath be exchanged with fresh plating solution in order to maintain the plating process.
One technique to address the accumulation of contaminants in a plating bath is disclosed in WO 99/19544). In this application, a portion of the plating solution is removed and replaced with fresh plating solution. However, even with the continuous addition and removal of solution, at some point the concentration of contaminants in the bath becomes too high and the plating process is degraded. The bath is then completely removed, treated as waste, and is replaced with a new plating solution.
Replacement of a plating bath is costly to production because it is a time consuming procedure that reduces the production throughput of the plating tool. The bath replacement also generates a significant amount of liquid waste that is hazardous to the environment and must be disposed of properly. It is expensive to dispose of such hazardous wastes in a controlled landfill. Pre-treating the bath to remove the harmful components so that the majority of the spent plating bath can be discharged as waste is complicated and difficult because it requires that the metal ions must be removed or reduced to a level sufficient to conform to national and local environmental discharge laws. Removal of metal ions requires additional equipment and chemicals and can include processes such as electrowinning. Additionally, the remainder of the organic additives or their residual components needs to be removed, typically by chemical precipitation or chemical oxidation, prior to discharge. Once the metal ions and organic contaminants have been removed, the remainder of the fluid is then treated as aqueous material by a plant's wastewater facility.
It is known that various process variables effect the efficiency and rates of oxidative degradation of organic contaminants in liquids. These include the presence of copper, cobalt, and iron ions, the concentration of hydroxide ions in solution, and the presence of radical scavengers or radical initiators such as carbonate and acetate ions. The addition of thermal, acoustic, or electromagnetic radiation also effects the efficiency and rate of oxidative processes.
It is also well known that hydrogen peroxide is an effective oxidizing agent especially when combined with ultraviolet light and heat. However the use of hydrogen peroxide results in dilution of the chemical which is being treated with water which is a solvent for the hydrogen peroxide and is also a byproduct of its chemical reaction with organic materials. A current accepted practice in the printed circuit board plating industry is to add excess volumes of hydrogen peroxide to spent plating solutions and to heat the solutions in order to oxidize the organic additive present.
In addition to the process variables known, various process and apparatus have been used to treat waste plating solutions containing organic contaminants. For example, U.S. Pat. No. 4,289,594 teaches a process for reducing the concentration of dissolved copper ion and organic complexing agent in an electroless copper plating waste solution. The process includes chemically reducing the copper ion to copper metal in a first tank to a concentration of less than 8 parts per million (ppm,) and then chemically precipitating the complexing agent after transfer of the solution to a second tank. The final step of the process requires contacting the solution with ozone gas in the presence of ultraviolet light (UV) to remove the trace levels of organics additives remaining in the bath. The remaining liquid material is then sent to a typical plant waste treatment system. In this invention the reduction of copper to less than 8 ppm is required to reduce the time required for the ozone oxidation process.
In a related method, U.S. Pat. No. 4,512,900 first chemically precipitates the copper ion in a spent plating bath to a concentration of less than 8 ppm. The reduction of copper ion concentration is required to reduce the oxidation process time. Hydrogen peroxide is used in a subsequent process step to reduce up to 60% the organic complexing agent remaining in the solution. In a preferred embodiment, the hydrogen peroxide is added to the solution following a chemical precipitation of the complexing agent and prior to treatment of the solution with ozone. The amount of hydrogen peroxide needed for this step is determined using an off-line organic carbon analyzer. The hydrogen peroxide can be combined with ultraviolet light and or heat up to a temperature of 90° C. and is used to rapidly reduce the remaining complexing agent concentration as compared to ozone and ultraviolet oxidation alone. In a final process step, the solution is pH adjusted to between 4 and 6 with sulfuric acid, and then ozone gas in the presence of ultraviolet light is sparged through the solution to further reduce the organic concentration. Thereafter, the remaining liquid material is conveyed to the plant waste processing system for disposal. The process is described as being faster than the use of ozone with ultraviolet light alone or the use of hydrogen peroxide alone.
The practice of using hydrogen peroxide in conjunction with ozone and UV light to oxidize and reduce the concentration of toxic organic compounds in copper ion free water has been disclosed in U.S. Pat. Nos. 4,849,114 and 4,792,407. U.S. Pat. No. 5,562,822 discloses the generation of hydroxyl radicals using ultraviolet light and ozone gas in the fluid and use of the hydroxyl radicals generated in removing contaminants from waste fluid streams.
U.S. Pat. No. 5,290,439 discloses an apparatus and use of an ultraviolet radiation device for purifying a flow of liquid. U.S. Pat. No. 5,043,080 discloses use of medium pressure polychromatic mercury arc lamps for treatment of contaminated ground waters.
All of the processes detailed above require the reduction of copper ion in the waste from the electroless plating bath before treatment to remove organic contaminants. This requires the use of additional chemicals for the metal ion precipitation reaction as well as multiple tanks for the different reactions. This adds complexity and cost to the processes and the loss of valuable chemical reagents which cannot be reused after treatment. In those processes where hydrogen peroxide is used there occurs a dilution of the treated liquid because of the addition of water in hydrogen peroxide, usually 70% or more by volume, along with water generated from the oxidation reaction process.
There is a need in the art for a method and system to reduce plating waste and to simplify the treatment of spent chemical plating solutions. The ability to reuse plating solutions after suitable treatment by process and system, preferably an in-line treatment process and system, would reduce chemical waste as well as reduce the time and cost of replacing spent plating solution with new solutions. A method and system that simplifies the use of hydrogen peroxide as an oxidant and minimizes the effect of dilution caused by water addition to the liquid to be treated is also needed. Because spent plating baths vary in the amount of contaminants that they contain; there is also a need to be able to detect and treat solutions containing different concentrations of contaminants and to indicate when such treatment processes have been completed.
The present invention provides a system and method for removing organic contaminants from a liquid plating solution. In direct contrast to the prior art, the present invention substantially maintains the copper ion concentration in the solution. Additionally, the treatment of the waste plating liquid in the present invention is reduced to a single step by combining the oxidants along with external energy sources. The destruction of organic contaminants in the plating solution is accomplished using sources of energy in combination with chemical oxidants. The present invention also minimizes the dilution of the liquid plating solution by liquid chemical oxidants by providing a controlled flow of gas into the liquid so as to control the evaporation of solvent from the liquid during the treatment process. The present invention removes the organic contaminants from the plating solution in a single pass process, and using suitable sensors indicates the end of the treatment process. The invention recycles the treated fluid for reuse rather than disposing of the fluid or trying to recover the complexing agent (cf. U.S. Pat. Nos. 5,091,070 and 4,734,175). Strategies for the removal of oxidants to prevent their release back into the process solution and the removal of chloride ions along with the degraded carbon products is addressed (described below). In addition, the current invention provides for metal ion and particle removal before the electroplating solution is returned to the system.